Despite enormous investments of financial and human resources, no cure exists for a variety of diseases. For example, cancer remains one of the major causes of death. A number of bioactive agents have been found, to varying degrees, to be effective against tumor cells. However, the clinical use of such antitumor agents has been highly compromised because of treatment-limiting toxicities.
In order to decrease drug-induced toxic side effects, antitumor agents have been encapsulated in liposomes. Liposomes are completely closed lipid bilayer membranes containing an entrapped aqueous volume. Liposomes may be unilamellar vesicles (possessing a single membrane bilayer) or multilamellar vesicles (onion-like structures characterized by multiple membrane bilayers, each separated from the next by an aqueous layer). The bilayer is composed of two lipid monolayers having a hydrophobic "tail" region and a hydrophilic "head" region. The structure of the membrane bilayer is such that the hydrophobic (non-polar) "tails" of the lipid monolayers orient toward the center of the bilayer while the hydrophilic (polar) "heads" orient toward the aqueous phase. The current state of the art is such that liposomes may be reproducibly prepared using a number of techniques.
Liposome encapsulation of various antitumor agents has been shown to decrease drug-induced toxic side effects while maintaining or, in some instances, increasing antitumor activity. Reduction of toxicity results from the ability of liposomes to decrease drug exposure, and subsequent damage, to susceptible tissues. The mechanism of the antitumor activity of entrapped drugs is less well understood, but may result from the capacity of liposomes to slowly release encapsulated drug into the circulation or alternatively passive targeting of liposomes and their contents to tumor sites. A problem, however, with the encapsulation of antitumor agents is that many of these drugs have been found to be rapidly released from liposomes after encapsulation.
An example of an antitumor agent is vincristine, which is a member of the Vinca alkaloid class and is derived from the periwinkle plant. It is an important anticancer drug in that it displays effectiveness against a wide variety of neoplasms including both the Hodgkin's and non-Hodgkin's lymphomas, acute lymphoblastic leukemia, embryonal rhabdomyosarcoma, neuroblastoma, breast carcinoma, and Wilm's tumor. It is a cell-cycle specific drug which arrests cell growth exclusively during metaphase by attaching to the growing end of microtubules and terminating their assembly. For this reason, it is advantageous to expose neoplastic cells to the drug for prolonged periods of time. This effect has been demonstrated in vitro by Jackson and Bender (Cancer Res. 39:4346, 1979), and has been confirmed using the murine L1210 leukemic cell line (Mayer etal., Cancer Chemother. Pharmacol. 33:17-24, 1993). The importance of this relationship in the treatment of human malignancies is supported by clinical trials where patients refractory to bolus vincristine therapy exhibited increased response rates when the drug was administered as a 5-day infusion.
Liposomal formulations of vincristine have been shown to exhibit reduced toxicity and enhanced efficacy compared to free drug. The antitumor activity of vincristine appeared to be dependent on the circulation lifetime of the encapsulated drug. Circulation longevity (of liposomally entrapped bioactive agent) in turn is dependent, in part, on the rate of agent release from liposomes in the blood. Therefore, enhancement of the retention of a bioactive agent in liposomes is desirable as it will increase the circulation lifetime of the encapsulated agent, thereby improving its therapeutic activity.
Thus, there is a need in the art for liposomal bioactive agent preparations with improved circulation longevity. The present invention fulfills this need, and further provides other related advantages.